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Susceptible (S) genes are those plant genes that facilitate pathogen infection and disease. The loss of function of these genes primarily interrupts the growth and development of invading pathogens...

Agrobacterium-mediated transformation of epicotyl segment has been used in Citrus transgenic studies. The approach suffers, however, from limitations such as occasionally seed unavailability, the low transformation efficiency of juvenile tissues and the high frequency of chimeric plants. Therefore, a suspension cell culture system was established and used to generate transgenic plants in this study to overcome the shortcomings. The embryonic calli were successfully developed from undeveloped ovules of the three cultivars used in this study, “Sweet orange”-Egyptian cultivar (Citrus sinensis), “Shatangju” (Citrus reticulata) and “W. Murcott” (Citrus reticulata), on three different solid media. Effects of media, genotypes and ages of ovules on the induction of embryonic calli were also investigated. The result showed that the ovules’ age interferes with the callus production more significantly than media and genotypes. The 8 to 10 week-old ovules were found to be the best materials. A cell suspension culture system was established in an H+H liquid medium. Transgenic plants were obtained from Agrobacterium-mediated transformation of cell suspension as long as eight weeks subculture intervals. A high transformation rate (~35%) was achieved by using our systems, confirming BASTA selection and later on by PCR confirmation. The results demonstrated that transformation of cell suspension should be more useful for the generation of non-chimeric transgenic Citrus plants. It was also shown that our cell suspension culture procedure was efficient in maintaining the vigor and regeneration potential of the cells.

Two closely related pathogens of Phytophthora are responsible for stem gummosis and root rot diseases in Citrus worldwide. In this study, a neglected local mandarin called Guanggan (Citrus reticulata) was identified to be strongly resistant to one of the pathogens, P. nicotianae. No visible symptom was seen on all leaf explants within one week or on 50% of in vivo inoculated stems within one month after inoculation with the pathogen. Drenching of the soil with zoosporangial and zoospore suspensions did not cause any reduction in tree growth and leaf chlorophyll content of pot-grown Guanggan seedlings. Quantitative real-time PCR analysis suggested that its resistance to P. nicotianae may be associated with high basal and induced expression of some defense-related genes, particularly the PR3 gene. In conclusion, C. reticulata var. Guanggan is a promising material that can be exploited to improve Phytophthora resistance in Citrus.

The endophytic bacterial and fungal profiles were comparatively studied between Citrus Huanglongbing (HLB) infected and uninfected healthy leaves of a mandarin cultivar, Shatangju (Citrus reticulata Blanco), by using the next generation sequencing technology. A total of 53 endophytic bacterial orders in12 phyla and 24 endophytic fungal orders in 2 phyla were detected in healthy, asymptomatic and symptomatic leaf midribs. It was found that the predominant endophytic bacteria belonged to Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes and the predominant endophytic fungi belonged to Ascomycota and Basidiomycota. Operational taxonomic unit (OTU) analysis showed that OTU_2 and OTU_6 were the core endophytic fungal OTUs, and OTU_10, OTU_4 and OTU_141 were the core endophytic bacterial OTUs. The richness and diversity of the endophytic microbes were affected by infection of the HLB bacterium, Candidatus Liberibacter asiaticus (CLas). The most significant change was the reduction in endophytic fungal OTUs following CLas infection. Eleven endophytic fungal OTUs and 24 endophytic bacterial OTUs were found to be associated with tree healthy status in citrus. Some of the healthy and HLB disease specific OTUs were discussed, with emphasis on their possible roles in HLB pathogenesis.

Root samples of 'Sanhu' red tangerine trees infected with and without Candidatus Liberibacter asiaticus (CLas) were collected at 50 days post inoculation and subjected to RNA-sequencing and isobaric tags for relative and absolute quantification (iTRAQ) to profile the differentially expressed genes (DEGs) and proteins (DEPs), respectively. Quantitative real-time PCR was subsequently used to confirm the expression of 16 selected DEGs. Results showed that a total of 3956 genes and 78 proteins were differentially regulated by HLB-infection. Among the most highly up-regulated DEPs were sperm specific protein 411, copper ion binding protein, germin-like proteins, subtilisin-like proteins and serine carboxypeptidase-like 40 proteins whose transcript levels were concomitantly up-regulated as shown by RNA-seq data. Comparison between our results and those of the previously reported showed that known HLB-modulated biological pathways including cell-wall modification, protease-involved protein degradation, carbohydrate metabolism, hormone synthesis and signaling, transcription activities, and stress responses were similarly regulated by HLB infection but different or root-specific changes did exist. The root unique changes included the down-regulation in genes of ubiquitin-dependent protein degradation pathway, secondary metabolism, cytochrome P450s, UDP-glucosyl transferases and pentatricopeptide repeat containing proteins. Notably, nutrient absorption was impaired by HLB-infection as the expression of the genes involved in Fe, Zn, N and P adsorption and transportation were significantly changed. HLB-infection induced some cellular defense responses but simultaneously reduced the biosynthesis of the three major classes of secondary metabolites, many of which are known to have anti-pathogen activities. Genes involved in callose deposition were up-regulated whereas those involved in callose degradation were also up-regulated, indicating that the sieve tube elements in roots were hanging on the balance of life and death at this stage. In addition, signs of carbohydrate starvation were already eminent in roots at this stage. Other interesting genes and pathways that were changed by HLB-infection were also discussed based on our findings.